High concentrations of neuropeptide Y (NPY) have been found in many chromaffin cells in adrenal glands of various species. Previously, we have found that, in addition to, authentic NPY, there is a NPY-like peptide in chromaffin cells of bovine adrenal glands. Interestingly, in the rat, this NPY-like peptide is present in the adrenal gland of the old rat but not in that of the young rat. This NPY-like peptide has not been purified to homogeneity and characterized by sequencing, other chemical analysis and carboxypeptidase hydrolysis. A high degree of homology with NPY, was observed and final confirmation of the structure determination is in progress. In order to study whether the NPY-like peptide may have a modulator role in catecholamine secretion from chromaffin cells, we have now established the primary culture of rat adrenal chromaffin cells. The characterization of the rat adrenal chromaffin cell during the course of the culture is in progress. Significance to Biomedical Research: A novel NPY-like peptide has been isolated from bovine adrenal glands. This peptide is present in the adrenal gland of the old rat in significant quantity but not in that of the young rat. Study on biological activities of this peptide may provide a new direction to study the adrenal function of aged animal.